This invention relates to a seal assembly for a roller cutter drill bit and more particularly to a seal assembly positioned between opposed sealing surfaces on a journal of the bit body and a rotary cutter mounted on the journal for rotation.
A roller cutter drill bit includes a roller cutter mounted for rotation on a bearing journal of the drill bit body, and bearing means mounting the roller cutter on the bearing journal. A lubrication system is provided to supply lubricant to the bearing means, and sealing means are normally mounted between the roller cutter and the bearing journal on opposed bearing surfaces to block the flow of lubricant outwardly past the bearing means and to block the passage of foreign matter such as drilling fluid with cuttings entrained therein inwardly to the bearing means. Rock drill bits typically have utilized roller cutters thereon mounted for rotation at relatively low speeds such as around 125 RPM, for example. As a result of the relatively high loads that may be placed on a drill bit during operation, a relatively large bearing clearance has been provided between the cutter and bearing journal such as around 0.001 inch to 0.004 inch per inch of diameter of the cutter, for example, which provides a relatively large gap which must be sealed at all times during rotation of the cutter for retaining the lubricant within the bearing means and preventing the entry of foreign matter past the sealing means. To accomplish this, seals heretofore have required a relatively large amount of deformation or squeeze when placed between the bearing surfaces on the journal and the cutter, such as around 0.035 inch, for example. When such seals are provided with such a large amount of deformation or squeeze, a substantial sliding friction is provided between the cutter and the seal, and when relatively high rotational speeds are involved, a substantial amount of heat will be generated thereby causing wear and decreasing the life of the seal.
Another problem frequently encountered in such rock drill bits is the wobbling of the cutter resulting from eccentric rotation of the cutter. The seal between the cutter and the journal must compensate for any eccentricity or wobbling and must seal at all times during rotation, even with the wobbling. Thus, a substantial amount of deformation or squeeze is required for the seal such as the aforementioned 0.035 inch in order to compensate for any wobbling of the roller cutter. High temperatures generated from sliding friction of an elastomeric seal might result in a permanent set or deformation of the seal and thereby reduce the ability of the seal to close the clearance between the journal and cutter and effectively seal at all times during rotation of the cutter.
Rock drill bits normally have a relatively large clearance provided between the cutter and the bearing journal to compensate for the relatively high operating temperatures encountered in severe conditions, such as 300.degree. F., for example, as the difference in the coefficient of expansion between the roller cutter and the associated journal and bushings requires a relatively large gap or clearance. Further, a high viscosity grease is usually employed for lubrication purposes and a relatively large clearance is needed for the grease to easily enter the bearings.
Heretofore, such as shown in U.S. Pat. Nos. 3,656,764 dated Apr. 18, 1974 and 4,466,621 dated Aug. 21, 1985, seal assemblies utilizing two separate but similar O-ring seals have been provided for drill bits between the journal and roller cutter thereon, one O-ring seal engaging a sealing surface on the roller cutter and the other O-ring seal engaging an opposed sealing surface on the journal. The O-ring seals illustrated have been separated by a floating ring maintaining a compression on both O-rings.
Another drill bit bearing seal assembly is shown in U.S. Pat. No. 3,467,448 dated Sept. 16, 1969 and this seal assembly includes a dynamic seal and a static seal separated by a metal ring. The dynamic seal comprises an O-ring mounted in a groove. The static seal, however, is bonded on one face to the metal ring separating the seals, and on an opposite face to a separate ring or metal sealing surface. Thus, only a relatively small portion of the periphery of the static ring is unbonded and permitted to deform or deflect thereby providing a very stiff or relatively rigid static seal.
An example of a roller cutter bearing seal assembly in which a pair of face seals separated by a metal ring are employed is illustrated by U.S. Pat. Nos. 4,344,629 dated Aug. 17, 1982 and 4,394,020 dated July 19, 1983. An O-ring which forms the dynamic seal is utilized only as a secondary seal until a primary lapped metal-to-metal seal is formed by wear during operation of the drill bit.